Fix #3855: Small Refactor of Playlist Helper

[Brandon-T]

Summary of Changes

• Refactor some of the code to make it cleaner and easier to read, understand, and maintain
• Refactor the AVURLAsset to check the playable status before even calling any playable loading functions
• Removed logging of WKScriptMessage on a delayed Main-Thread. I do NOT know if this will fix a possible crash because I can never reproduce it, but this one is a guess at a possible solution (though, the previous code did access it only on main-thread, this is just better anyway).

This pull request fixes #3855

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()

Test Plan:

• Test on a very slow network using the Link Conditioner
• Refresh the page, attempt to scroll immediately and keep scrolling for up to 10 seconds after the page fully loads and the playlist bar shows.
• If scrolling is smooth as the previous build, we're good.

Screenshots:

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • release-notes/(include|exclude)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue is assigned to a milestone (should happen at merge time).